“Wood chips” are small pieces of wood of generally uniform size and shape produced from fresh tree trunks and limbs by mechanical treatment (chipping) with sharp tools such as knives. Wood chips typically have a parallelepiped shape with six rhombus faces and a low and consistent thickness compared to length and width dimensions. Length along the grain direction is generally 5 to 50 mm (0.2 to 2 inches). The cross-grain ends are smoothly cut (chipped) at an oblique angle to induce chip formation at appropriate thickness. Particular sizes and shapes of chips have long been standardized as feedstocks for pulp and paper production, particleboard manufacture, and other industrial uses. For example, chips for pulp and paper manufacturing generally maximize intact fibers and fiber bundles along the grain between chipper-cut ends. Several generations of chipping equipment have been developed to produce such wood-chip feedstocks. However, conventional wood chips are not optimal as direct feedstocks for many biomass conversion processes.
In the case of biofuel production it is preferable to maximize diffusion and heat-transfer distances both across and along the grain. It is also preferable to cut across the fiber bundles rather than preserve their natural length. In addition, many biofuel processes require small feedstock particle sizes. Re-comminution of wood chips has introduced processing concerns. Milling and grinding affects biomass particle shape as well as size. The hammer mill process tends to break wood chips multiple times along the grain, initially into narrow sticks (aka, pin chips) and eventually into narrower fiber bundles that tend to retain the full chip length. Likewise, grinding, wet milling, and crushing tend to separate fibers without cutting across the grain. Such comminution machines often require dry chips, at 15% wet wood basis (wwb) or less, yet drying woody biomass below its fiber saturation point, approximately 40-45% wwb (˜30% dry basis), is an energy intensive process that may increase feedstock recalcitrance during conversion processing.
Thus, it would be advantageous to provide comminuted wood-chip particles with properties more favorable to biomass conversion processing.